The invention relates to a method for operating a drive train of a motor vehicle, wherein the drive train comprises a drive motor and an automatic or automated transmission connected between the drive motor and the drive wheels.
At present, vehicles which are equipped with an automatic transmission or an automated transmission can usually be operated in at least two different transmission operating modes. If the driver changes to the D transmission gear position, the transmission control occurs in a fully automatic first operating mode (=automatic mode), i.e. the transmission control unit selects the optimal transmission gear for the current operating state on the basis of evaluated vehicle parameters and changes thereto. In addition to this automatic mode, the driver can change by means of corresponding use of the gear lever to a manual mode, in which a down-shift or up-shift typically only occurs when the driver requests an up-shift or down-shift by way of an operating element (for example, by pressing a selector lever forward or backward) provided therefor. Alternatively, the automatic mode can also be left and changed to manual mode with the correspondingly requested gear through actuation of an operating element typically disposed near the steering wheel, through which the driver can request an up-shift or down-shift. Such a so-called automatic transmission with Steptronic is known, for example, from the current BMW 5-Series.
It is further known that motor vehicles consume very little fuel when they “coast,” that is, when they use their kinetic energy to roll as far as possible without being slowed down by the drag torque of the engine. Already known from DE 10 2009 057 551 A1 is a method for operating a drive train in a motor vehicle, wherein the internal combustion engine is decoupled from the drive wheels of a moving motor vehicle in the presence of predetermined operating conditions, thus is transferred into coasting operation or coasting mode.
Finally, vehicles with automatic transmissions are already on the market, in which wherein the automatic transmission can be operated in an automatic mode or a normal mode, and wherein in automatic mode the transmission can be shifted to a coasting mode of the drive train in the presence of predefined operating conditions, in which the operative connection between the internal combustion engine and the drive wheels is separated. If during coasting mode a manual up-shift or down-shift by the driver occurs by use of the operating elements provided therefor, the coasting mode is left and changed to the normal mode of the drive train, i.e. the operative connection between the internal combustion engine and the drive wheels is restored.
The object of the invention is to provide an improved method in terms of comfort for operating a drive train of a vehicle with automatic transmission or automated transmission, which can be operated in coasting mode under certain conditions.
This and other objects are achieved by a method for operating a drive train of a motor vehicle, wherein the drive train comprises a drive motor and an automatic or automated transmission connected between the drive motor and the drive wheels. The transmission automatically changes gear in an automatic mode. The transmission in manual mode is changed to a higher or lower gear dependent on an up-shift or down-shift requested by the driver by way of an operating element. At least in automatic mode, a shift occurs in the presence of defined operating conditions to a coasting mode of the drive train, in which the operative connection between the drive motor and the drive wheels is disengaged. The coasting mode is left when a driver requests a down-shift and shifts to the manual mode of the transmission. In an up-shift requested by the driver during coasting mode, the coasting mode is maintained in automatic mode.
The method according to the invention, as well as the advantageous embodiments thereof, can be carried out by way of an executed algorithm or a corresponding disposition of components in a control device provided therefor, for example in a transmission control device.
In the invention, it is assumed that the vehicle includes an automatic or automated transmission connected between the drive motor and the drive wheels, and that the transmission can be operated both in automatic mode, in which the gear is changed automatically, i.e. independently of the driver, and in manual mode, in which the gear is changed to a higher or lower gear dependent on an up-shift or down-shift requested by the driver by way of an operating element. Further, the method for operating the drive train of the vehicle is fundamentally configured such that at least in automatic operation, the transmission can be changed to a coasting mode, in which the operating connection between the drive wheels and the drive motor is separated in the presence of defined operating conditions (for instance, neither the gas pedal nor the brake pedal is actuated). This can take place in that, in coasting mode, an automatic or automated clutch connecting the drive wheels with the drive motor is opened and/or the transmission is shifted into a neutral position. An automated clutch is to be here understood as all couplings (or transmission shifting mechanisms) of the (automatic) transmission which are required to engage the individual gears and connect the drive motor with the transmission output such that a frictional connection exists between the drive unit and the drive wheels. In addition, the drive motor can be turned off during coasting mode.
In principle, the coasting mode can also be permitted only when the driver desires a predetermined, for example particularly energy efficient, driving mode. This desire for an energy-efficient driving mode can be detected by analysis of driver behavior or the vehicle behavior or by evaluating and recognizing a driver-selectable operating mode (for example, an ECO mode).
The coasting mode is then usually left when the driver actuates the gas pedal (or the brake pedal) to at least a given degree. Likewise, the coasting mode is left when the driver actuates an operating element provided for shifting the transmission stage back to manual mode.
However, if in coasting mode the driver actuates in a corresponding manner an operating element provided for up-shifting the transmission stage in manual mode, it can be inferred that at this time the driver at least desires no deceleration of the motor vehicle. Since, however, a termination of the coasting mode is usually accompanied—if only slightly—by a deceleration of the vehicle due to the drag torque, the method according to the invention provides that during an up-shift requested by the driver in coasting mode, the coasting mode of the drive train is maintained (when no other termination condition for coasting mode is met), i.e. the up-shift request does not affect the control of the transmission or the drive train.
Advantageously, the coasting mode is then maintained in automatic mode during an up-shift request from the driver during coasting mode if the up-shift occurs by way of an operating element disposed in the region of the steering wheel, in particular a rocker switch located there. If, by means of actuation of the transmission selector lever, the driver changes through corresponding actuation to manual mode while also requesting an up-shift, the coasting mode can hereby be directly left and changed to manual mode (incl. an up-shift), corresponding to the request. The driver thus has the possibility, by means of a corresponding actuation of an operating element, to actively leave the coasting mode and to execute an up-shift, if he so wishes.
In order to inform the driver that although his up-shift request is detected, this will not be carried out due to the presence of coasting mode, a shift request display is activated at least for a predetermined time interval in the case of a requested up-shift during coasting mode and when coasting mode remains. After completion of this shift request display, the driver can again be displayed the original display which was displayed at the point in time before the up-shift request was active (for instance, that he is currently in coasting mode). This shift request display can be identical to a display which appears when the vehicle is in manual mode and he requests an up-shift and this will be implemented.
As already described earlier, it can be assumed during a down-shift request of the driver during coasting mode that he anticipates a deceleration of the vehicle, or even expects this. In this case, coasting mode is left and possibly simultaneously changed to the manual mode of the vehicle. Advantageously, in a requested down-shift during coasting mode, the coasting mode is left by closing the operative connection between the drive motor and drive wheels, and the gear virtually engaged during coasting mode is maintained in the change to manual operation. This means that even in this case no gear shift occurs, as a deceleration of the motor vehicle, which the driver would also expect during a down-shift, is already caused through the ending of the coasting mode and the reestablishment of the operative connection associated therewith.
Advantageously, the method according to the invention for operating a drive train of the motor vehicle motor is additionally designed such that, under the assumption that the motor vehicle or transmission is in manual mode and that the highest gear is thereby already engaged, in a requested up-shift (which cannot be implemented as the transmission is already in the highest gear), the drive train is changed to coasting mode. The driver should thereby be made to feel that due to his up-shift request in the highest gear, the drive train control or regulation is altered in such a way as would already be expected with a further up-shift in similar manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.